A. Technical Field
The present invention relates to a crosslinked polymer particle and its production process and use.
B. Background Art
In recent years, synthetic water-absorbing materials, which are called water-absorbent resins, have been developed and are being put to practical use not only as constituent materials of sanitary materials such as sanitary napkins and disposable diapers, but also for purposes which need the water-absorbent resin, for example, industrial purposes such as sealing materials, dewfall-preventive materials, freshness-keeping materials, and solvent-dehydrating materials, and water-holding purposes in such as tree planting, agriculture, and horticulture.
As to the above water-absorbent resin, many ones as obtained by giving a crosslinked structure to hydrophilic high-molecular compounds are known, and examples thereof include: crosslinked polymers of partially neutralized polyacrylic acids; hydrolyzed products of starch-acrylonitrile graft polymers; neutralized products of starch-acrylic acid graft polymers; saponified products of vinyl acetate-acrylic acid ester copolymers; hydrolyzed products of acrylonitrile- or acrylamide copolymers, or their crosslinked polymers; and crosslinked polyethylenimine.
Examples of processes for giving a crosslinked structure to high-molecular compounds include: a process in which a high-molecular compound is obtained and then post-crosslinked by adding thereto a crosslinking agent during or after polymerization; and a process in which a predetermined amount of an internal-crosslinking agent is beforehand added to a monomer to carry out polymerization to obtain a high-molecular compound, and a crosslinking reaction of the resultant high-molecular compound is carried out simultaneously with or after the polymerization. These crosslinking reactions may be carried out either in a solution or suspension state, but the crosslinking reaction in the suspension state has advantages in that a crosslinked polymer can be obtained in the form of approximately spherical particles, and further in that it is easy to control the size of such particles. However, the crosslinking reaction in the suspension state has various problems in that a surfactant needs to be used to stably obtain a suspension and remains in the resultant crosslinked polymer particles even after polymerization and is therefore still contained further in products. For example, the use of the resultant crosslinked polymer particles for disposable diapers has problems in that the surfactant flows from the polymer particles due to external pressure or with time to decrease the surface tension of the absorbed liquid, thus causing the deterioration of the water absorption properties of the crosslinked polymer, or in that some surfactants are unfavorable in view of the safety. In addition, there are also problems in that the surfactant lowers the heat resistance. It may be possible to decrease the content of the surfactant to some extent by washing the crosslinked polymer particles as obtained using the surfactant, but it is impossible to make the content zero, and the process becomes complicated because the number of the steps increases by one.
An object of the present invention is to provide a process in which o surfactant is used, or even if the surfactant is used the amount thereof is as small as possible, thus stably obtaining a suspension to carry out a crosslinking reaction.
To solve the above problems, the present invention provides the following constitutions:
(1) A production process for a crosslinked polymer particle, comprising the steps of:
forming a suspension in which liquid drops of a first phase are dispersed in a second phase by stirring both phases in the presence of an inorganic particle, wherein the first phase includes a high-molecular compound and a crosslinking agent, and wherein the first phase and the second phase are insoluble in each other, and wherein the affinity of the inorganic particle for water is different from that of the high-molecular compound; and
carrying out a crosslinking reaction.
(2) A production process according to constitution (1) above, comprising the steps of:
forming a water-drop-in-oil type suspension by stirring a water phase and an oil phase in the presence of a hydrophobic inorganic particle, wherein the water phase includes a hydrophilic high-molecular compound and a crosslinking agent; and
carrying out a crosslinking reaction.
(3) A production process according to constitution (2) above, wherein the hydrophilic high-molecular compound is a cationic, anionic, or nonionic polymer.
(4) A production process according to constitution (2) or (3) above, wherein the hydrophobic inorganic particle is hydrophobic fumed silica.
(5) A production process according to any one of constitutions (1) to (4) above, wherein the amount of the inorganic particle relative to the first phase including the high-molecular compound and the crosslinking agent is in the range of 0.001xcx9c10 weight %.
(6) An approximately spherical crosslinked polymer particle, which is free from a surfactant.
(7) An approximately spherical crosslinked polymer particle according to constitution (6) above, being used as either one or both of an anionic and a cationic water-swellable crosslinked polymer particle, wherein the anionic and the cationic water-swellable crosslinked polymer particle are combined to give a pair of crosslinked polymer particles which are favorable for absorption of electrolyte solutions, and wherein 50xcx9c100% of functional groups in the polymer molecule of the anionic water-swellable crosslinked polymer particle are acid groups, and 50xcx9c100% of functional groups in the polymer molecule of the cationic water-swellable crosslinked polymer particle are basic groups.
(8) An approximately spherical crosslinked polymer particle according to constitution (7) above, being used as either one or both of the anionic and the cationic water-swellable crosslinked polymer particle, wherein the anionic water-swellable crosslinked polymer particle is at least one member selected from the group consisting of crosslinked polyacrylic acids, crosslinked polymaleic acids, and their derivatives, and wherein the cationic water-swellable crosslinked polymer particle is at least one member selected from the group consisting of crosslinked polyethylenimines, crosslinked polyallylamines, and their derivatives.
(9) An absorbent article, comprising the approximately spherical crosslinked polymer particle as recited in constitution (6) above.
These and other objects and the advantages of the present invention will be more fully apparent from the following detailed disclosure.